Place flour and milk into the Thermomix bowl and blend 5 sec/speed 6. Cook 15 min/80°C/ speed 4. Pour into bowl, cover with plastic wrap, pushing plastic down onto surface of mixture. Cool completely.
In the clean and dry Thermomix bowl, add sugar and mill 15 sec/speed 9. Add butter and chop 5 sec/speed 7. Aerate butter and sugar 1–2 min/ Butterfly/speed 3/MC off. Scrape down sides of bowl as necessary.
Add vanilla paste and beat 6 sec/ Butterfly/speed 4.
Add cooked and cooled roux mixture and continue to beat for around 2–3 min/ Butterfly/speed 3 until very smooth and white. Cover and refrigerate for 15 minutes. Use immediately.
To make the Chocolate Ermine Frosting, simply add the cocoa powder to the first process, cooking it with the roux. Then proceed as directed.